Parametric transmit waveform generator for medical ultrasound imaging system

ABSTRACT

A medical diagnostic ultrasonic imaging system includes a transmit waveform generator that uses stored parameters to completely define an arbitrarily complex transmit waveform. Preferably, the stored parameters define an envelope function and a modulation function in a piecewise fashion using a number of sets of quadratic parameters. These quadratic parameters are used to calculate the desired envelope function and modulation function in the log domain, and the envelope and modulation functions are combined in the log domain and then converted to the linear domain. Multiple separate transmit waveforms may be combined in a single channel, and individual channels may be combined prior to application to the transducer elements.

BACKGROUND

[0001] The present invention relates to medical diagnostic ultrasonicimaging systems, and in particular to digital transmit waveformgenerators adapted for such systems.

[0002] In the prior art, digital transmit beamformers are known that usea memory such as a RAM to store a sampled version of the desiredtransmit waveform envelope. The data stored in RAM can be a complexbaseband envelope sampled at the Nyquist frequency. See Cole, U.S. Pat.No. 5,675,554, assigned to the assignee of this invention. In this case,signal processing techniques are then used to interpolate, filter, andmodulate the envelope to form the desired ultrasonic transmit waveform.In some cases, multiple simultaneous transmit beams are generated inreal time in a time-interleaved manner. See the above-identified Colepatent. As another alternative, the desired ultrasonic transmit waveformcan be stored directly in RAM.

[0003] The methods described above require a memory size that increaseslinearly with the time duration of the transmit waveform. For longtransmit waveforms such as coded excitation transmit pulses, the numberof samples stored in memory can exceed currently available RAM sizes.For a given RAM size, the number of samples required for each transmitwaveform limits the number of concurrent RAM transmit waveforms. In somecases, this can limit the number of distinct transmit waveforms perbeam, or may require reloading the RAM on a line-by-line basis, whichmay adversely affect the frame rate.

[0004] The process of interpolating, filtering and modulating aNyquist-sampled baseband signal can in some cases limit the finalbandwidth and frequency of the ultrasonic transmit waveform. Inaddition, interpolating and filtering a Nyquist-sampled signal canresult in spurious signals due to non-ideal filtering. This effect isespecially apparent when the carrier frequency is verniered from thecenter of the filter pass band.

[0005] Time interleaving multiple transmit beams is hardware efficient,but it utilizes a tradeoff between the number of transmit beams, thebandwidth, and/or the center frequency. In some implementations at thehighest center frequency and the highest permitted bandwidth only asingle transmit beam is allowed per channel.

SUMMARY

[0006] By way of introduction, the preferred embodiment described belowcalculates ultrasonic transmit waveforms by storing a set of parametersthat defines both an envelope function and a modulation function for thedesired ultrasonic transmit waveform, and then calculating theultrasonic transmit waveform in real time based on the set ofparameters. The envelope function is preferably a smoothly rising andfalling function, such as a Gaussian function.

[0007] In this embodiment the parameters entirely define the ultrasonictransmit waveform, and for this reason the stored parameters efficientlyuse system memory, even for transmit waveforms of long duration.

[0008] The preferred transmit waveform generator comprises a transmitwaveform calculator that calculates the waveforms in the log domain,thereby minimizing the need for multipliers. Preferably, the waveformcalculator comprises a plurality of accumulators that are operative toform respective quadratic functions in real time. These quadraticfunctions define the respective ultrasonic transmit waveform functionsin the log or linear domain.

[0009] The disclosed embodiment combines multiple transmit waveformsusing combiners that comprise of plurality of inputs and outputs, andmultiplexers that switch transmit waveforms from respective single orcombined transmit waveform generators to desired transducer channels.

[0010] This section is intended as a brief introduction, and it is notintended to limit the scope of the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a generalized block diagram of an accumulator of thetype used in the preferred embodiment of this invention.

[0012]FIGS. 2a-2 k are timing diagrams used in describing the operationof the accumulator of FIG. 1.

[0013]FIG. 3 is a block diagram of a medical diagnostic ultrasonicimaging system that incorporates a preferred embodiment of thisinvention.

[0014]FIG. 4 is a block diagram of one of the transmit waveformgenerators 120 of FIG. 3.

[0015]FIG. 5 is a more detailed block diagram of the envelopeaccumulator 154 and the phase accumulator 156 of FIG. 4.

[0016]FIG. 6 is a more detailed diagram of the envelope/phase processor158 of FIG. 4.

[0017]FIG. 7 is a circuit diagram of the envelope stage 170 of FIG. 6.

[0018]FIG. 8 is a circuit diagram of the phase stage 172 of FIG. 6.

[0019]FIG. 9 is a circuit diagram of the log-linear stage 178 of FIG. 6.

[0020]FIG. 10 is a circuit diagram of the beam summer 122 of FIG. 3.

[0021]FIG. 11 is a circuit diagram of the channel summer 124 of FIG. 3.

[0022]FIG. 12 is a circuit diagram of the gain and clip stage 126 ofFIG. 3.

[0023]FIG. 13 is a circuit diagram of the encoder 130 of FIG. 3.

[0024] FIGS. 14-18 are graphs used to describe the generation of aGaussian transmit waveform.

[0025] FIGS. 19-23 are graphs used to describe the generation of alinear FM Gaussian transmit waveform.

[0026]FIGS. 24 and 25 are time and frequency domain graphs of a Hanningpulse, respectively.

[0027]FIGS. 26 and 27 are time and frequency domain graphs of a Hammingpulse, respectively.

[0028]FIGS. 28 and 29 are time and frequency domain graphs of abroadband pulse, respectively.

[0029]FIG. 30 is a block diagram of the transmitters 102 configured in asingle-channel mode.

[0030]FIGS. 31 and 32 are block diagrams of the transmitters 102configured in a multi-channel mode.

[0031]FIG. 33 is a plot of a waveform envelope produced by the system ofFIG. 32.

[0032] FIGS. 34-37 are block diagrams of single-channel (FIG. 34) andmulti-channel (FIGS. 35-37) configurations for the transmitters 102.

[0033]FIG. 35 shows 128 transmit processing channels applied to a 64element transducer, and FIGS. 36, 37 show 128 transmit processingchannels applied to any contiguous block of 64 elements of a 128 elementtransducer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] General Discussion

[0035] The specific examples described below generate ultrasonictransmit waveforms efficiently by calculating quadratic functions thatdefine the envelope and the modulation function of the waveform. Thesequadratic functions are then combined, converted to the linear domainwhere appropriate, and used to drive respective transducer elements. Thefollowing discussion presents the basic mathematical framework of theapproach implemented in the example of FIGS. 3-13.

[0036] A Gaussian ultrasonic transmit waveform x(t) can be expressed asfollows: $\begin{matrix}{{x(t)} = {{{Re}\left\{ {{Ae}^{{- {\pi {({1 - {i\rho}})}}}{(\frac{t - \tau}{T_{c}})}^{2}}e^{{i2\pi}\quad {f_{c}{({t - \tau})}}}} \right\}} = {{Ae}^{- {\pi {(\frac{t - \tau}{T_{c}})}}^{2}}{\cos \left( {{2\pi \quad {f_{c}\left( {t - \tau} \right)}} + {\pi \quad {\rho \left( \frac{t - \tau}{T_{c}} \right)}^{2}}} \right)}}}} & \left( {{Eq}.\quad 1} \right)\end{matrix}$

[0037] where

[0038] A=gain,

[0039] ρ=time-bandwidth product,

[0040] T_(c)=Gaussian pulse duration,

[0041] f_(c)=carrier frequency,

[0042] τ=pulse delay.

[0043] y(t), the logarithm (base 2) of the ultrasonic waveform x(t), istherefore:

y(t)=log₂(x(t))=g(t)+log₂(cos(2πθ(t))),   (Eq. 2)

[0044] and

x(t)=2^(y(t))Δ.

[0045] In Eq. 2, the function g(t) may be sampled at discrete timesnΔ_(t,) where Δ_(t) is the interval between samples and n is the samplenumber. In addition, τ may be quantized to Δ_(t) resolution such that τis equal to n₉₆ ·Δ_(t). With these conventions g_(n) is equal to g(t) attime nΔ_(t), and g_(n) can be expressed as follows: $\begin{matrix}{{g_{n} = {{g\left( {n\quad \Delta_{t}} \right)} = {{{\log_{2}(A)} - {\pi \quad {\log_{2}(e)}\left( \frac{{n\quad \Delta_{t}} - \left( {n_{\tau}*\Delta_{t}} \right)}{T_{c}} \right)^{2}}} = {{a_{0}n^{2}} + {b_{0}n} + c_{0}}}}},} & \left( {{Eq}.\quad 3.} \right)\end{matrix}$

[0046] where $\begin{matrix}{{a_{0} = {{- \pi}\quad {\log_{2}(e)}\left( \frac{\Delta_{t}}{T_{c}} \right)^{2}}},} & \left( {{Eq}.\quad 4} \right) \\{{b_{0} = {2\quad \pi \quad {\log_{2}(e)}\left( \frac{\Delta_{t}}{T_{c}} \right)^{2}n_{\tau}}},} & \left( {{Eq}.\quad 5} \right) \\{c_{0} = {{\log_{2}(A)} - {\pi \quad {\log_{2}(e)}\left( \frac{\Delta_{t}}{T_{c}} \right)^{2}{n_{\tau}^{2}.}}}} & \left( {{Eq}.\quad 6} \right)\end{matrix}$

[0047] Similarly, the function θ(t) of Eq. 2 when sampled at discretetimes nΔ_(t) may be expressed as follows: $\begin{matrix}{{\theta_{n} = {{\theta \left( {n\quad \Delta_{t}} \right)} = {{{f_{c}\left( {{n\quad \Delta_{t}} - \left( {n_{\tau}*\Delta_{t}} \right)} \right)} + {\frac{\rho}{2}\left( \frac{{n\quad \Delta_{t}} - \left( {n_{\tau}*\Delta_{t}} \right)}{T_{c}} \right)^{2}}} = {{a_{1}n^{2}} + {b_{1}n} + c_{1}}}}},} & \left( {{Eq}.\quad 7} \right)\end{matrix}$

[0048] where $\begin{matrix}{{a_{1} = {\frac{\rho}{2}\left( \frac{\Delta_{t}}{T_{c}} \right)^{2}}},} & \left( {{Eq}.\quad 8} \right) \\{{b_{1} = {{f_{c}\Delta_{t}} - {{\rho \left( \frac{\Delta_{t}}{T_{c}} \right)}^{2}n_{\tau}}}},} & \left( {{Eq}.\quad 9} \right) \\{c_{1} = {{{- f_{c}}\Delta_{t}n_{\tau}} + {\frac{\rho}{2}\left( \frac{\Delta_{t}}{T_{c}} \right)^{2}{n_{\tau}^{2}.}}}} & \left( {{Eq}.\quad 10} \right)\end{matrix}$

[0049] The embodiment of FIGS. 3-13 uses iteration techniques togenerate g_(n) and θ_(n) efficiently as piecewise quadratic functions ofthe form

y _(n) =an ² +bn+c.   (Eq. 11)

[0050] The starting value y_(o) is set equal to c, and each subsequentvalue y₁, y₂, y₃ . . . is determined iteratively according to Eq. 12:

y _(n+1) =y _(n) +T _(n) , y _(o) =c,   (Eq. 12)

[0051] where T_(o)=a+b and each subsequent value T_(n+1)=T_(n)+2a.

[0052] The envelope function (log domain) g_(n) and the modulation phaseθ_(n) for an ultrasonic transmit waveform can be expressed as follows:

envelope (log domain): g _(n) =a ₀ n ² +b ₀ n+c ₀,   (Eq. 13)

[0053] and

modulation phase: θ_(n) =a ₁ n ² +b ₁ n+c ₁.   (Eq. 14)

[0054] The modulated envelope (log domain) y_(n) is therefore equal to

modulated envelope (log domain): y _(n) =g _(n) +LUT _(cos) [θ _(n)]=yint _(n) +yfrac _(n),   (Eq. 15)

[0055] where $\begin{matrix}{{{{LUT}_{\cos}(k)} = {\log_{2}\left( {\cos \left( {\frac{\pi}{2}k} \right)} \right)}},{k = {\left\lbrack {{0\ldots \quad N_{table}} - 1} \right\rbrack/N_{table}}},} & \left( {{Eq}.\quad 16} \right)\end{matrix}$

[0056] and the modulated envelope (linear domain) {circumflex over(x)}_(n) is therefore equal to

modulated envelope (linear domain): x _(n)=2^(yint) ^(_(n)) LUT _(pwr)[−yfrac _(n)],   (Eq. 17)

[0057] where $\begin{matrix}{{{{LUT}_{pwr}(k)} = 2^{- k}},{k = {\frac{\left\lbrack {{0\ldots \quad N_{table}} - 1} \right\rbrack}{N_{table}}.}}} & \left( {{Eq}.\quad 18} \right)\end{matrix}$

[0058] In Eq. 15 and 17, yi{circumflex over (n)}t_(n) and yfrâ_(n)represent the integer and fractional components of ŷ_(n), respectively.

[0059] FIGS. 1-2 and the following discussion explain an accumulatorimplementation of Eq. 11 and 12, and the embodiment of FIGS. 3-13provides one implementation of Eq. 13, 14, 15 and 17 that usesaccumulators like that of FIG. 1. The above describes generation of aGaussian pulse, but, by concatenating multiple fitted second ordersegments, any arbitrary transmit pulse can be generated.

[0060] Accumulator Timing

[0061]FIGS. 1 and 2 will be used to explain the timing and operation ofan accumulator 10 of the type used in the embodiment of FIGS. 3-13. Asshown in FIG. 1, the accumulator includes a memory 12 that stores a setof four parameters for each segment or zone of a selected ultrasonictransmit waveform. The accumulator 10 implements the equationy_(n)=an²+bn+c, and the four parameters for each zone include the valuesof a, b, and c for that zone as well as the value of z, the width ornumber of clock cycles of the zone.

[0062] In the following discussion, the parameters a_(n), b_(n), c_(n),z_(n) indicate the stored parameters for zone n, and the parametersa_(n,i,) b_(n,i), c_(n,i) indicate calculated values for cycle i of zonen that are used in implementing the quadratic equation for zone ndefined by the parameters a_(n), b_(n), c_(n).

[0063] Returning to FIG. 1, the parameters a_(n), b_(n), c_(n), z_(n)are stored in the memory 12 in compressed form as an integer exponentand a mantissa, and they are expanded in a shift register 14. Theparameters a_(n), b_(n), c_(n,) z_(n), for a given zone n are read outof the memory 12 in series (FIG. 2b), and they are routed by themultiplexers 16, 22, 26 and stored in the registers 18, 24, 30,respectively, during respective clock cycles, as shown in FIGS. 2f, 2 g,2 h, 2 i, 2 j, and 2 k. The summer 20 sums the output U of the register18 with the output T of the register 24, and the summer 28 sums theoutput S of the register 30 with the output T of the register 24 (whenthe multiplexer 26 is in the logic 0 state).

[0064]FIGS. 2c, 2 d, and 2 e show the values of the signals U, T and S,respectively, at various clock cycles. Using the notation of Eq. 11 and12 above, these signal values are shown in Table 1. TABLE 1 Zone 0 (a₀,b₀, c₀, z₀ = 4) Zone 1 (a₁, b₁, c₁, z₁ > 6) t i b_(0,i) c_(0,i) b_(1,i)c_(1,i) 0 0 a₀ + b₀ c₀ 1 1 3a₀ + b₀ a₀ + b₀ +c₀ 2 2 5a₀ + b₀ 4a₀ + 2b₀ +c₀ 3 3 7a₀ + b₀ 9a₀ + 3b₀ + c₀ 4 0 a₁ + b₁ c₁ 5 1 3a₁ + b₁ a₁ + b₁ + c₁6 2 5a₁ + b₁ 4a₁ + 2b₁ + c₁ 7 3 7a₁ + b₁ 9a₁ + 3b₁ + c₁ 8 4 9a₁ + b₁16a₁ + 4b₁ + c₁ 9 5 11a₁ + b₁ 25a₁ + 5b₁ + c₁ 10  6 13a₁ + b₁ 36a₁ +6b₁ + c₁

[0065] Note that the output S of the accumulator 10 (corresponding tothe values of c_(n,i) in Table 1) implements a piecewise quadraticequation, in which the values of S in each piecewise zone or segment aredetermined by the quadratic parameters a_(n), b_(n), c_(n) stored in thememory 12 for the zone n. The accumulator 10 is efficient to implement,because multiplication operations are not required, and the hardwareused to implement the described shifting and adding functions isrelatively simple.

Specific Examples

[0066] FIGS. 3-13 provide detailed information regarding one preferredembodiment of this invention. As shown in FIG. 3, a medical diagnosticultrasonic imaging system 100 includes a plurality of transmitters 102that supply ultrasonic transmit waveforms via amplifiers 103 and atransmit/receive switch 104 to individual transducer elements of atransducer 106. The transducer 106 forms ultrasonic pressure waves in aregion being imaged in response to these high voltage signals, andechoes from these pressure waves impinge upon the transducer 106. Theresulting echo signals are passed via the transmit/receive switch 104 toa receiver 108 that beamforms, detects and demodulates the echo signalsto form received beam signals that are processed by an image processor110 for display on a display 112.

[0067] Depending upon the application, the ultrasonic transmit waveformssupplied by the transmitters 102 may be processed (e.g. delayed, phased,apodized, gain-calibrated, delay-calibrated, and phase-calibrated) tocause the ultrasonic waves emitted by the transducer 106 to be focusedalong selected scan lines, though this is not a requirement for allembodiments. The elements 102-112 can take any suitable form, and thepresent invention is suitable for use with the widest variety of suchdevices.

[0068] Continuing with FIG. 3, each transmitter 102 includes multipletransmit waveform generators 120, each generating a respectiveultrasonic transmit waveform. Selected ones of the transmit waveformsare summed in a beam summer 122, and selected ones of the summed beamsare combined in a channel summer 124. The resulting combined transmitwaveforms are gain controlled and clipped in respective stages 126,filtered in respective filters 128 and encoded in respective encoders130.

[0069] The following discussion will concentrate on the transmitwaveform generators 120, and any suitable alternative can be used forthe remaining elements 122-130, depending upon the application. Thechannel summer 124, the filters 128, and the encoders 130 are optional,and may be deleted in some embodiments. Similarly, the beam summer 122is not required in all embodiments.

[0070] As shown in FIG. 4, each of the transmit waveform generators 120includes RAM memories 150, 152 for storing envelope and phaseparameters, respectively. Envelope parameters from the memory 150 areapplied to an envelope accumulator 154, and phase parameters from theRAM 152 are applied to a phase accumulator 156. The accumulators 154,156 generate envelope functions and phase functions, respectively, andthese functions are applied to an envelope/phase processor 158. Theenvelope function is in the log domain in one mode of operation. Theenvelope/phase processor 158 combines the envelope function with thephase function, in the log domain in this mode of operation, convertsthe result to the linear domain, and supplies as an output an ultrasoundtransmit waveform that is applied to the beam summer 122 of FIG. 3.

[0071]FIG. 5 provides details of construction of one preferredembodiment of the envelope accumulator 154 and the phase accumulator156. Note that the accumulators 154, 156 in this embodiment operate asdescribed above in conjunction with FIGS. 1 and 2a-2 k. In this mode ofoperation, the output signal env of the envelope accumulator 154 definesthe envelope of the transmit waveform in log domain, and the outputsignal phs of the phase accumulator 156 defines the phase functionθ(t).The output signals env_zone_width and phs_zone_width correspond to the zparameter discussed above, and are used to control the timing at whichthe next set of parameters is read out of the memories 150, 152 suchthat the signals env, phs are both constructed in a piecewise manner,with each piece corresponding to a respective segment or zone of theultrasonic transmit waveform in the quadratic form defined by therespective set of stored parameters.

[0072]FIG. 6 shows a more detailed view of the envelope/phase processor158 of FIG. 4. The processor 158 includes an envelope stage 170 thatreceives the signal env from the envelope accumulator 154 along with apre-computed gain signal. The pre-computed gain signal may represent anapodization signal, a calibration signal, a scaling signal, or anycombination of these and other signals. Combining the various gain termsis efficient because the gain terms are simply added in the log domain.The envelope stage 170 operates in two states, depending upon the stateof the map_select signal. In a first state, the signal env is addeddirectly with the pre-computed gain signal in the log domain. In thismode the signal corresponds to a Gaussian envelope in the linear domain.In a second state, the env signal is converted from linear domain to logdomain and then added to the pre-computed gain signal. In this mode thesignal corresponds to a quadratic envelope in the linear domain. FIG. 7illustrates one preferred circuit for implementing the envelope stage170 of FIG. 6.

[0073] The processor 158 of FIG. 6 also includes a phase stage 172 thatreceives as inputs the phs signal from the phase accumulator 156 and apre-computed phase signal. The pre-computed phase signal may representcalibration values or other desired offsets for the modulation phaseangle of the transmit waveform, such as a phase adjustment toapproximate a fine delay. The phase stage 172 supplies two outputsignals: logcos, which is the log base 2 of the cosine of the sum of thesignal phs and the pre-computed phase, and sign, which is the sign oflogcos. FIG. 8 shows a circuit diagram for one preferred form of thephase stage 172.

[0074] The processor 158 of FIG. 6 includes a summer 176 that combinesthe log output of the envelope stage with the logcos output of the phasestage. In effect, the summer 176 operates in log domain to modulate theenvelope signal supplied by the envelope stage 170 with a modulationsignal supplied by the phase stage 172. The output of the summer 176 isapplied to a log-to-linear stage 178 that converts the modulatedenvelope signal in log domain supplied by the summer 176 to lineardomain. The log-to-linear stage 178 receives another input from a gate174 that defines the sign of the resulting output signal. FIG. 9 is acircuit diagram for one preferred form of the log-to-linear stage 178.

[0075] The output of the log-to-linear stage 178 is an ultrasonictransmit waveform in linear domain that is applied as an output of thetransmit waveform generator 120.

[0076] With reference to the equations of the foregoing generaldiscussion, the envelope accumulator 154 of FIGS. 4 and 5 generates thesignal env according to Eq. 13, and the phase accumulator 156 of FIGS. 4and 5 generates the signal phs according to Eq. 14. Similarly, thelookup table LUT 0 of FIG. 8 implements Eq. 16, and the lookup table LUT1 of FIG. 9 implements a function closely related to that of Eq. 18.

[0077] It should be noted that the transmit waveform generator 120calculates the ultrasonic transmit waveform based on a parametricdescription of the transmit waveform. The embodiment described above hasbeen optimized for the calculation of linear FM modulated Gaussianpulses. In addition, this embodiment has the flexibility to generatearbitrary transmit waveforms by fitting both the envelope amplitude andthe modulating phase angle to any desired number of piecewise secondorder sections. The accumulators described above are a particularlyefficient hardware implementation. Because the computations areperformed in the log domain, all multipliers are replaced by adders andsmall lookup tables. This efficiency allows dedicated hardware to beused for each transmit waveform.

[0078] The embodiment described above provides a number of importantadvantages:

[0079] 1. It may be efficiently configured for modulated Gaussiantransmit waveforms, including chirps. Only eight parameters are used todefine any piecewise segment of the transmit waveform, independent ofthe desired length of the segment.

[0080] 2. It provides a very wide pulse bandwidth, since the samplingfrequency is at RF and filter effects can therefore be completelyavoided.

[0081] 3. It completely avoids extraneous aliased signal components atlower center frequencies, because no up-sampling techniques arerequired.

[0082] 4. It allows high bandwidth pulses to be matched to the idealcase with excellent accuracy.

[0083] 5. As described below, up to four separate ultrasonic transmitwaveforms can be combined to form a multi-beam transmit waveform, andthis is independent of the center frequency or the bandwidth that isused for individual transmit waveforms. This is the case because theindividual transmit waveforms are generated in parallel.

[0084] 6. It provides high time delay resolution for all centerfrequencies and it retains fine delay resolution via phasingadjustments.

[0085] 7. It uses a single, highest system clock in operation and isable to modulate to an arbitrary carrier frequency without deleteriousfilter effects.

[0086] 8. It uses multiple piecewise quadratic functions to approximatearbitrary envelope and phase functions.

[0087] Though less efficient, alternative embodiments of this inventioncan be implemented using multipliers in the linear domain rather thanadders and lookup tables in the log domain as described above.

[0088] A preferred implementation for the beam summer 122 of FIG. 3 isshown in FIG. 10. The gates 190, 192, 194, 196 can be used to pass anyselected ones of four ultrasonic waveform signals to the summers 198,200, 202 and thereby to the output. The output signal beamsum is thusthe combination of any one, any two, any three, or all four of the inputtransmit waveforms. The term “beam channel” will be used here to refereither to the output of one of the generators 120 or the output of thebeam summer 122.

[0089]FIG. 11 provides more detail regarding one implementation of thechannel summer 124 of FIG. 3. The channel summer in-this embodimentreceives four beam channel inputs and supplies four output signals, eachdestined for a respective transducer element. The term “transducerchannel” will be used here to refer to such output signals, at any stagealong the path from the channel summer 124 to the associated transducerelement.

[0090] The channel summer 124 includes summers 210, 212, 214 thatprovide summation signals to multiplexers 216, 218, 220, 222. Thesemultiplexers have three states as indicated. In state 0, each of thefour input channels C0, C1, C2, C3 is simply applied without alterationto the respective output terminal D0, D1, D2, D3, and all of the outputterminals D0, D1, D2, D3 are active. When the multiplexers are in state1, the sum of the signals on channels C0 and C2 is applied in parallelto output terminals D0 and D2, the sum of input channels C1 and C3 isapplied in parallel to output terminals D1 and D3, and one outputterminal is active in each subset D0, D2; D1, D3. When the multiplexersare in state 2, all four of the input channels C0, C1, C2 and C3 aresummed, this sum is applied in parallel to all four of the outputterminals D0, D1, D2, D3, and only one of the output terminals D0, D1,D2, D3 is active. The channel summer 124 is an example of a combiner.Other examples include multipliers or dividers that combine two or morebeam channels, time interleavers, and time concatenators.

[0091]FIGS. 12 and 13 provide additional information regarding onepreferred implementation of the stage 126 and the encoder 130 of FIG. 3,respectively.

[0092] By way of illustration, the circuits described above for thetransmitter 102 can preferably be implemented in an ASIC that includesthe elements 120-130 for four transducer channels per package.Preferably, the output resolution is plus or minus 256 codes, and themaximum envelope sampling rate is equal to 56 MHz. The total number oftransmit waveforms per beam channel can be varied between 1 and 4, andthe maximum transmit pulse length for a real/complex envelope is greaterthan 8192 sampled at 56 MHz.

Transmit Waveform Examples

[0093] The system described above in connection with FIGS. 3-13 cangenerate a wide variety of transmit waveforms. This section provides afew examples, as well as examples for modified versions of theillustrated system.

[0094] a. Single-Channel Gaussian Transmit Waveforms

[0095] FIGS. 14-18 relate to a first example, in which a single transmitwaveform generator 120 generates a Gaussian transmit waveform for eachrespective beam channel. In this example, the beam summer 124 selectsonly a single generator 120 for each respective beam channel, and thechannel summer 124 passes the signal on each beam summer output directlyto the respective transducer channel.

[0096]FIG. 14 shows one example of the output signal env of the envelopeaccumulator 154 described above in conjunction with FIG. 5, for thefollowing coefficient values:

[0097] a=−0.001967;

[0098] b=0.251798;

[0099] c=−8.057529.

[0100]FIG. 15 shows one example of the output signal phs of the phaseaccumulator of FIG. 5 for the following coefficient values:

[0101] a=0.000000;

[0102] b=0.082500;

[0103] c=−4.000000.

[0104]FIG. 16 shows the resulting signals logcos and sign of FIG. 6, forthe case where precompute phase is equal to zero; and FIG. 17 shows thelog domain output of the summer 176 of FIG. 6. The resulting lineardomain ultrasonic transmit waveform (the beam 0 signal of FIG. 6) isshown in FIG. 18. This waveform has a Gaussian envelope with a graduallyrising leading edge and a gradually falling trailing edge.

[0105] b. Single-Channel, Linear-FM, Gaussian Transmit Waveforms

[0106] FIGS. 19-23 correspond to FIGS. 14-18, respectively, for adifferent set of quadratic coefficients. In this case, the coefficientvalues used for the envelope function of FIG. 19 and the phase functionof FIG. 20 are as follows: envelope Phase a = −0.004426 a = 0.001953 b =0.566545 b = −0.187500 c = −18.129441 c = 4.000000

[0107] The resulting logcos and sign signals are shown in FIG. 21 andthe resulting sum of the envelope and modulation functions (log domain)is shown in FIG. 22. The resulting ultrasonic transmit waveform (lineardomain) is a linear-FM, Gaussian pulse, as shown in FIG. 23.

[0108] c. Single-Channel Transmit Waveforms with Envelope Parameterizedby Functions Based on Cosines

[0109] The examples described above use quadratic functions toapproximate the desired functions, but other parametric functions may beused. For example the envelope function env(t) of the ultrasonicwaveform may be parameterized using cosine-based functions as follows:$\begin{matrix}{{{{env}(t)} = {\sum\limits_{k = 0}^{3}\quad {a_{k}{\cos \left( {2\quad {\pi \cdot {k\left( \frac{t}{T} \right)}}} \right)}}}},{{\frac{- T}{2} \leq t \leq \frac{T}{2}};}} \\{{= 0},\left| t \middle| {> {\frac{T}{2}.}} \right.}\end{matrix}$

[0110]FIGS. 24 and 25 show time and frequency domain plots,respectively, of a COS parameterized Hanning envelope generated with thefollowing values of the coefficients a_(k): 0.5, 0.5, 0, 0. FIGS. 26 and27 show time and frequency domain plots, respectively, of a COSparameterized Hamming envelope generated with the following values ofa_(k): 0.54, 0.46, 0, 0. FIGS. 28 and 29 show time and frequency domainplots, respectively, of a COS parameterized broad band pulse envelopegenerated with the following values of a_(k): 0.999448,1.911456,1.078578, 0.183162.

[0111] d. Single-Channel Modes of Operation

[0112] In one single-channel mode of operation, the transmitter 102assigns a single transmit waveform generator 120 to each beam channel,and each beam channel is applied to a single transducer element via asingle transducer channel. FIG. 30 provides a block diagram for thismode of operation, using the elements of FIG. 4. In one example, thetransmit waveforms X₀(t), X₁(t) for transducer channels 0 and 1 are bothpulses with Gaussian envelopes as described above.

[0113] e. Multi-Channel Modes of Operation

[0114] In multi-channel modes of operation, the transmitter assignsmultiple transmit waveform generators 120 to each active transducerchannel. For example, the transmit waveform X₀(t) for channel 0 may takethe form of a modulated sinc function. In one embodiment, X₀(t) takesthe form${X_{0}(t)} = {\frac{\sin \left( {\pi \quad {Bt}} \right)}{\left( {\pi \quad {Bt}} \right)} \cdot {{\cos \left( {2\quad \pi \quad f_{o}t} \right)}.}}$

[0115] The logarithm (base 2) of X₀(t) is therefore expressed asfollows:

log₂(X ₀(t))=log₂(sin(πBt))+log₂(cos(2πf _(o) t))−log₂(πBt).

[0116] This embodiment may be implemented by combining two beams asshown in FIG. 31.

[0117] This is an example of multi-channel operation in which twotransmit waveforms (each generated by a separate generator 120) overlapin time. That is, the generators 120 operate during the same timesegment, and the transmit waveform at any time during the segment isobtained by combining transmit signals from two or more beam channels.

[0118] In another multi-channel mode of operation, two or moregenerators 120 operate during consecutive, non-overlapping time segmentsto generate a longer transmit waveform. This embodiment is shown inblock diagram form in FIG. 32, and the resulting transmit waveformenvelope is shown in FIG. 33. Note that only the first generator 120 aassociated with a first beam channel operates during a first timesegment Sa, and only the second generator 120 b associated with a secondbeam channel operates during the second time segment Sb. In thisexample, the RAMs 150, 152 each hold only three zones of parameters, butthe overall transmit waveform has six separate zones: three zonesgenerated by the generator 120 a and three zones generated by thegenerator 120 b.

[0119] FIGS. 34-37 illustrate several modes of operation for theembodiment of FIG. 3, which for purposes of discussion is assumed tohave 128 beam channels and 128 transducer channels. In this example, theoutputs of the beam summer 122 will be referred to as respective beamchannels, and the outputs of the channel summer 124 will be referred toas transducer channels. In the mode of FIG. 34, beam channels 0-63 areused to drive transducer elements 0-63 of a 64-element transducer, andbeam channels 64-127 are idle. This is an example of single-channeloperation.

[0120] In the mode of FIG. 35, beam channels n and (n+64) are combinedto drive transducer element n via transducer channel n (0≦n≦63). This isan example of multi-channel operation.

[0121]FIGS. 36 and 37 provide two examples of multi-channel operationfor the case where 128 beam channels are used with a 128-elementtransducer. In FIG. 36, only elements 0-63 are active, and each elementn is coupled with two beam channels: n and (n+64). In FIG. 37, onlyelements 64-127 are active, and each element (n+64) is coupled with twobeam channels: n and (n+64). Any contiguous block of 64 elements can bedriven (simultaneously) by two beam channels per element.

[0122] The channel combiners 124 of FIGS. 35-37 may combine therespective beam channel signals in many ways, including by adding,multiplying or dividing the signals on the respective beam channels.

[0123] Thus, a single-channel waveform is determined as a function ofthe processing capability of a single beam channel, and a multi-channelwaveform is determined as a function of the processing capability of twoor more beam channels. The embodiments of FIGS. 35-37 provide theadvantage that the processing power of beam channels that wouldotherwise be idle is used to form more complex or longer transmitwaveforms on the active transducer channels.

[0124] Of course, it should be understood that many changes andmodifications can be made to the preferred embodiments described above.For example, the parametric waveform generation techniques describedabove can be implemented with multipliers rather than accumulators, andthe channel summing and beam summing techniques described above can beused with other sources of ultrasonic transmit waveforms. The term“source” as used in this context is intended broadly to encompass anysource of ultrasonic transmit waveforms, including those generateddirectly from memory, and those generated by modulating a stored orinterpolated envelope using signal processing techniques, for example.

[0125] In an alternative embodiment, the ultrasonic transmit waveform iscalculated in real time from a set of parameters that define thewaveform directly, rather than defining the waveform as an envelopefunction that is modulated by a modulation function. Orthogonalfunctions such as Walsh functions and Hermite functions are examples offunctions that may be used.

[0126] As used herein, the term “ultrasonic transmit waveform” isintended to refer to an RF frequency ultrasonic waveform that is appliedto a transducer, at any stage in the signal path between the output ofthe transmit waveform generator 120 and the input to the transducer 106.The term “transmit waveform” is used to refer either to a piece or azone of a total pulse, or the entire pulse.

[0127] The term “combiner” is intended broadly to include summers,multipliers, lookup tables and the like, whether operating in parallelor in time-interleaved fashion.

[0128] The term “calculate” is intended to include calculation in boththe log domain and the linear domain, but to exclude signal processingtechniques operating on sampled baseband envelopes.

[0129] The term “set” is used broadly to encompass one or more, and theterm “accumulator” is used broadly to encompass adders. The term“logarithm” or “log” is intended to encompass logarithms in any base.

[0130] The foregoing detailed description has discussed only a few ofthe many forms that this invention can take. For this reason, thisdetailed description is intended by way of illustration and notlimitation. It is only the following claims, including all equivalents,that are intended to define the scope of this invention.

1. In a medical ultrasound imaging system, a method for generating atransmit waveform comprising: (a) storing a set of parameters thatdefines both an envelope function and a modulation function for anultrasonic transmit waveform having more than two levels; and (b)calculating the ultrasonic transmit waveform in real time based on theset of parameters.
 2. The method of claim 1 wherein the envelopefunction for the ultrasonic transmit waveform is non-rectangular.
 3. Themethod of claim 1 wherein the envelope function for the ultrasonictransmit waveform gradually rises during an initial portion of theultrasonic transmit waveform.
 4. The method of claim 1 wherein theenvelope function for the ultrasonic transmit waveform gradually fallsduring a final portion of the ultrasonic transmit waveform.
 5. Themethod of claim 1 wherein the set of parameters defines the envelopefunction in log domain.
 6. The method of claim 5 wherein the envelopefunction comprises at least one quadratic envelope function, and whereinthe set of parameters comprises at least one set of quadratic envelopecoefficients.
 7. The method of claim 1 wherein the modulation functioncomprises at least one quadratic phase function, and wherein the set ofparameters comprises at least one set of quadratic modulationcoefficients.
 8. The method of claim 1 wherein (a) comprises storing theset of parameters in a digital memory.
 9. The method of claim 8 wherein(b) comprises calculating the transmit waveform with a digitalprocessor.
 10. The method of claim 1 wherein the envelope functioncomprises a plurality of quadratic envelope functions, each associatedwith a respective zone of the transmit waveform, and wherein the set ofparameters comprises a plurality of respective sets of quadraticenvelope coefficients.
 11. The method of claim 1 wherein the modulationfunction comprises a plurality of quadratic phase functions, eachassociated with a respective zone of the transmit waveform, and whereinthe set of parameters comprises a plurality of respective sets ofquadratic phase coefficients.
 12. A transmit waveform generator for amedical ultrasound imaging system, said transmit waveform generatorcomprising: (a) a memory storing a set of parameters that defines bothan envelope function and a modulation function for an ultrasonictransmit waveform having more than two levels; and (b) a transmitwaveform calculator operative to calculate the transmit waveform in realtime based on the set of parameters stored in the memory.
 13. Theinvention of claim 12 further comprising a set of digital to analogconverters responsive to the transmit waveforms.
 14. The invention ofclaim 12 wherein the set of parameters defines the envelope function inlog domain.
 15. The invention of claim 12 wherein the set of parametersdefines the envelope function in linear domain.
 16. The invention ofclaim 14 wherein the envelope function comprises at least one quadraticenvelope function, and wherein the set of parameters comprises at leastone set of quadratic envelope coefficients.
 17. The invention of claim12 wherein the modulation function comprises at least one quadraticphase function, and wherein the set of parameters comprises at least oneset of quadratic phase coefficients.
 18. The invention of claim 12wherein each memory stores the respective set of parameters in digitalform.
 19. The invention of claim 18 wherein each calculator comprises arespective digital processor.
 20. The invention of claim 12 wherein theenvelope function comprises a plurality of quadratic envelope functions,each associated with a respective zone of the transmit waveform, andwherein the set of parameters comprises a plurality of respective setsof quadratic envelope coefficients.
 21. The invention of claim 12wherein the modulation function comprises a plurality of quadratic phasefunctions, each associated with a respective zone of the transmitwaveform, and wherein the set of parameters comprises a plurality ofrespective sets of quadratic phase coefficients.
 22. In a medicalultrasound imaging system, a method for generating a transmit waveformcomprising: (a) storing a set of parameters that entirely define anultrasonic transmit waveform having more than two levels; (b) generatingthe ultrasonic transmit waveform in real time based on the set ofparameters.
 23. The method of claim 22 wherein the set of parameterscomprises envelope parameters that define an envelope function for theultrasonic transmit waveform.
 24. The method of claim 23 wherein theenvelope parameters comprise polynomial coefficient parameters.
 25. Themethod of claim 23 wherein the envelope parameters comprise coefficientsof a set of base functions associated with the envelope function. 26.The method of claim 22 wherein the set of parameters comprisesmodulation parameters that define a modulation function for theultrasonic transmit waveform.
 27. The method of claim 26 wherein themodulation parameters comprise polynomial coefficient parameters. 28.The method of claim 26 wherein the modulation parameters comprisecoefficients of a set of base functions associated with the modulationfunction.
 29. The method of claim 22 wherein the set of parameterscomprises at least first parameters associated with a first segment ofthe ultrasonic transmit waveform and second parameters associated with asecond segment of the ultrasonic transmit waveform, and wherein (b)comprises generating the ultrasonic transmit waveform in a piecewisemanner.
 30. The method of claim 29 wherein (b) comprises generating anenvelope function for the ultrasonic transmit waveform in a piecewisemanner.
 31. The method of claim 29 wherein (b) comprises generating amodulation function for the ultrasonic transmit waveform in a piecewisemanner.
 32. The method of claim 22 wherein the set of parameters definesthe ultrasonic transmit waveform directly.
 33. The method of claim 32wherein the parameters comprise coefficients of a set of base functions.34. A transmit waveform source for a medical ultrasound imaging systemcomprising a plurality of transducer channels, said transmit waveformsource comprising: a plurality of transmit waveform generators, eachoperative to generate at least one respective transmit waveform; aplurality of combiners, each combiner comprising a plurality of inputscoupled with respective ones of the generators and an output; amultiplexer comprising a plurality of inputs coupled to respective onesof the combiners and a plurality of outputs coupled to respective onesof the transducer channels.
 35. The invention of claim 34 wherein themultiplexer in a first state connects respective generators to a set ofn transducer channels and in a second state connects respectivegenerators to a reduced set of m transducer channels, where m<n.
 36. Theinvention of claim 34 wherein the combiner comprises a summer.
 37. Theinvention of claim 34 wherein the combiners comprises a plurality ofgates, each gate operative to selectively gate the respective input onand off.
 38. The invention of claim 34 wherein the combiner comprises amultiplier.
 39. In a medical ultrasound imaging system, a method forgenerating transmit pulse waveforms comprising: (a) providing a set oftransmitters comprising n transducer channels, each transducer channeladapted for connection to a respective transducer element; and n beamchannels, each beam channel comprising a respective set of transmitwaveform generators; (b) generating first multi-level transmit pulsewaveforms on the n transducer channels with said n beam channels in afirst mode of operation, wherein each of the beam channels is associatedwith a separate respective one of the transducer channels; and (c)generating second multi-level transmit pulse waveforms on a subset ofthe n transducer channels with said beam channels in a second mode ofoperation, wherein multiple ones of the beam channels contribute to eachof the second transmit waveforms.
 40. The method of claim 39 whereineach first transmit waveform is characterized by a respective Gaussianenvelope, and wherein each second transmit waveform is characterized bya respective non-Gaussian envelope.
 41. The method of claim 39 whereinthe subset includes n/2 transducer channels.
 42. The method of claim 39wherein the subset includes n/4 transducer channels.
 43. The method ofclaim 39 wherein each second transmit waveform is a function of multiplesegments, each segment generated by a respective beam channel.
 44. Themethod of claim 43 wherein at least two of the segments of one of thesecond transmit waveforms overlap in time.
 45. The method of claim 43wherein at least two of the segments of one of the second transmitwaveforms do not overlap in time.
 46. In a medical ultrasound imagingsystem, a method for generating transmit waveforms comprising: (a)providing a set of transmitters comprising n transducer channels, eachtransducer channel adapted for connection to a respective transducerelement; and n beam channels, each beam channel comprising a respectiveset of transmit waveform generators; (b) generating first transmitwaveforms on the n transducer channels with said n beam channels in afirst mode of operation, wherein each of the beam channels is associatedwith a separate respective one of the transducer channels; and (c)generating second transmit waveforms on a subset of the n transducerchannels with said beam channels in a second mode of operation, whereinmultiple ones of the beam channels contribute to each of the secondtransmit waveforms.
 47. The method of claim 46 wherein each firsttransmit waveform is characterized by a respective Gaussian envelope,and wherein each second transmit waveform is characterized by arespective non-Gaussian envelope.
 48. The method of claim 46 wherein thesubset includes n/2 transducer channels.
 49. The method of claim 46wherein the subset includes n/4 transducer channels.
 50. The method ofclaim 46 wherein each second transmit waveform is a function of multiplesegments, each segment generated by a respective transmit beam channel.51. The method of claim 50 wherein at least two of the segments of oneof the second transmit waveforms overlap in time.
 52. The method ofclaim 50 wherein at least two of the segments of one of the secondtransmit waveforms do not overlap in time.
 53. A transmit waveformgenerator for a medical ultrasonic imagining system, said generatorcomprising: a transmit waveform calculator operative to calculate in alog domain and in realtime a first signal indicative of a logarithm ofat least a component of an ultrasonic transmit waveform; and a converterresponsive to the calculator and operative to convert a signal thatvaries as a function of the first signal to a second signal indicativeof the ultrasonic transmit waveform in linear domain.
 54. The inventionof claim 53 wherein the calculator comprises a summer operative in thelog domain to effect apodization multiplication in the linear domain.55. The invention of claim 53 wherein the calculator comprises a summeroperative in the log domain to effect transmit gain multiplication inthe linear domain.
 56. The invention of claim 53 wherein the calculatorcomprises a summer operative in the log domain to effect envelope andmodulation multiplication in the linear domain.
 57. The invention ofclaim 53 wherein the calculator comprises a summer operative in the logdomain to effect gain calibration multiplication in the linear domain.58. The invention of claim 53 wherein the calculator comprises a summeroperative in the log domain to effect multiplication in the lineardomain.
 59. The invention of claim 53 wherein the calculator calculatesthe logarithm in log base
 2. 60. A transmit waveform generator for amedical ultrasonic imaging system, said generator comprising: a transmitwaveform calculator comprising a plurality of accumulators, eachaccumulator operative to generate a respective output signal in a logdomain and in real time, each output signal indicative of a logarithm ofa respective component of an ultrasonic transmit waveform; and aprocessor operative to combine the output signals in the log domain toform a combined signal.
 61. The invention of claim 60 furthercomprising: a converter responsive to the combined signal and operativeto convert the combined signal to linear domain.
 62. The invention ofclaim 60 wherein the calculator implements a quadratic function.
 63. Atransmit waveform generator for a medical ultrasonic imaging system,said generator comprising: a transmit waveform calculator comprising atleast one accumulator responsive to a plurality of polynomialcoefficients to generate a polynomial function in real time, thepolynomial function indicative of at least a component of an ultrasonictransmit waveform.
 64. The invention of claim 63 wherein the polynomialfunction is indicative of at least the component of the ultrasonictransmit waveform in log domain.
 65. The invention of claim 63 whereinthe polynomial coefficients comprise quadratic coefficients, and whereinthe polynomial function comprises a quadratic function.
 66. Theinvention of claim 64 wherein the polynomial coefficients comprisequadratic coefficients, and wherein the polynomial function comprises aquadratic function.
 67. In a medical ultrasound imaging system, a methodfor allocating transmit waveform processing, said method comprising: (a)providing a set of transmitters comprising n transducer channels, eachtransducer channel adapted for connection to a respective transducerelement; and n beam channels, each beam channel comprising a respectiveset of transmit waveform generators; (b) trading off (1) the number ofbeam channels generating multi-level pulse waveforms for each activetransducer channel with (2) the number of transducer channels that aresimultaneously active.
 68. In a medical ultrasound imaging system, amethod for allocating transmit waveform processing, said methodcomprising: (a) providing a set of transmitters comprising n transducerchannels, each transducer channel adapted for connection to a respectivetransducer element, and n signal generators, each signal generatorassociated with a separate respective transducer channel in asingle-channel mode of operation; and (b) trading off (1) the number ofsignal generators generating multi-level pulse waveforms for each activetransducer channel with (2) the number of transducer channels that aresimultaneously active.